1. Field of the Invention
The present invention relates to a driving device for fluorescent tube and, more particularly, to a fluorescent tube driving device capable of driving a fluorescent tube to emit light with high brightness.
2. Description of Related Art
Conventionally, a driving device is required for starting and driving a fluorescent tube to emit light. Typically, the starting voltage of fluorescent tube is about several hundred volts, and thus the driving device must provide such a high voltage output. As such, the quality of the output voltage provided by the driving device determines the brightness of light emitted from the fluorescent tube and the stability of fluorescent light. A conventional driving device for fluorescent tube is implemented by a winding-based electronic oscillation circuit for providing the high voltage output. However, such a winding-based electronic oscillation circuit is not able to provide a high step-up voltage for output with high conversion efficiency due to the limitation of the oscillation frequency. Furthermore, the starting time is more than 0.5 to 0.8 second, which is deemed to be relatively long. Recently, the liquid crystal display has become more and more popular. The liquid crystal display generally requires a back light module using a cold cathode fluorescent lamp (CCFL) as the light source. Such a back light module is required to have a high energy conversion efficiency. As such, it becomes an important issue to have a driving device for driving the fluorescent tube to steadily emit fluorescent light with high brightness.
The object of the present invention is to provide a driving device for fluorescent tube capable of providing a high conversion efficiency and high step-up voltage ratio to emit light with high brightness, and providing a short starting time to protect the fluorescent tube.
To achieve the object, there is provided with a driving device for a fluorescent tube, which comprises: a high frequency oscillator having an input terminal connected to a power source, and an output terminal for outputting a high frequency AC signal; a pulse width modulator (PWM) having an input terminal connected to the output terminal of the high frequency oscillator, and an output terminal for outputting a PWM harmonic frequency signal; a first power switch connected to the output terminal of the pulse width modulator for being turned off during a positive half-cycle of the PWM harmonic frequency signal and being turned on during a negative half-cycle of the PWM harmonic frequency signal; a second power switch connected to the output terminal of the pulse width modulator for being turned on during the positive half-cycle of the PWM harmonic frequency signal and being turned off during the negative half-cycle of the PWM harmonic frequency signal; and a piezoelectric transformer having a primary winding and a secondary winding, the primary winding having two input terminals connected to the first power switch and the second power switch, respectively, and a center terminal connected to the output terminal of the pulse width modulator.
Other objects, advantages, and novel features of the invention will become more apparent from the detailed description when taken in conjunction with the accompanying drawings.